1. Field of the Invention
The invention relates to a method for controlling a hydrodynamic retarder in a motor vehicle having a drive motor that can be disengaged via a disconnect clutch.
2. Description of the Related Art
It is known to equip motor vehicles with a hydrodynamic retarder that is also referred to as a hydrodynamic brake in order to generally be able to brake the motor vehicle wear free, in addition to braking with the so-called operating brake (friction brake) that is subject to wear. A hydrodynamic retarder of this type is known in various construction forms. They all have in common that at least two bladed wheels or rings together form a working chamber that can be filled with and drained of a working medium, in order to transfer torque via a hydrodynamic circulation flow of the working medium in the working chamber from the first bladed wheel to the second bladed wheel and to thereby decelerate the first bladed wheel.
It is known that hydrodynamic retarders still exert a certain braking effect upon the shaft, driving them or upon another component even in the so-called non-braking mode, in other words when no braking effect is to be created with them, since a circulatory flow of air is created in the working chamber that transfers an accordingly low residual braking torque between the blade wheels. Such a residual braking torque leads to increased fuel consumption of the drive motor of the motor vehicle since said motor operates against the residual braking torque.
In addition to constructive disadvantages and additional manufacturing and maintenance costs the integration of a disconnect clutch that engages during the transition from the non-braking mode to the braking-mode and disengages during the transition from the braking mode to the non-braking mode means that the build-up of a braking torque with the hydrodynamic retarder following a relevant request by the vehicle operator to turn on (initiation)—for example by operating a braking actuator or via a driver assist system can only occur when the disconnect clutch is engaged and a volume of working medium according to the requested braking torque has been introduced into the working chamber, whereby an appropriate amount of air has to be displaced from the working chamber which retards the introduction. In order to avoid that the disconnect clutch works already during synchronization against a braking torque that is provided by the retarder, engaging of the disconnect clutch and introduction of the working medium into the working chamber occurs consecutively with or without overlapping time periods, starting initially with engaging of the disconnect clutch. The necessary time period until the desired braking torque is available is an addition therefore, of the two consecutively provided time periods for engaging the disconnect clutch and for introduction of the working medium into the working chamber.
In order to be able to provide the braking torque requested by the vehicle operator or a driver assist system faster after initiating a braking procedure it has already been suggested in DE 10 2007 024 698 A1 to preventatively engage the disconnect clutch when the drive motor of a motor vehicle is in a so-called overrun operation even if no initiation of a braking procedure by the vehicle operator or via a driver assist system has occurred. If then, subsequently the vehicle operator or the driver assist system requests braking with the hydrodynamic retarder, in other words requests the transition from non-braking mode to braking mode then the working chamber of the hydrodynamic retarder merely needs to be filled with working medium. The delay between the initiation of the braking mode and the actual braking torque provided by the hydrodynamic retarder through engagement of the disconnect clutch is eliminated.
In practical applications it has been shown that the suggested solution results in a satisfactory response time of the hydrodynamic retarder; that however, the service life of the disconnect clutch is reduced.
It is therefore the objective of the current invention to improve the method described at the beginning in such a way that, when providing the braking torque with the hydrodynamic retarder without delay after initiation of the braking mode the service life of the clutch can be increased.